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    <h1 class="post-title">
      undefined与null
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    <h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h2><p>Undefined 和 Null 是 Javascript 中两种特殊的原始数据类型(Primary Type)，它们都只有一个值，分别对应 undefined 和 null ，下面就来详细说说两者的异同。</p>
<h2 id="undefined"><a href="#undefined" class="headerlink" title="undefined"></a>undefined</h2><p>undefined 的字面意思就是：未定义的值 。这个值的语义是，希望表示一个变量最原始的状态，而非人为操作的结果 。 这种原始状态会在以下 4 种场景中出现：</p>
<h3 id="声明了一个变量，但没有赋值"><a href="#声明了一个变量，但没有赋值" class="headerlink" title="声明了一个变量，但没有赋值"></a>声明了一个变量，但没有赋值</h3><pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token keyword">var</span> foo<span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>foo<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span></span></code></pre>

<p>访问 foo，返回了 undefined，表示这个变量自从声明了以后，就从来没有使用过，也没有定义过任何有效的值，即处于一种原始而不可用的状态。</p>
<h3 id="访问对象上不存在的属性"><a href="#访问对象上不存在的属性" class="headerlink" title="访问对象上不存在的属性"></a>访问对象上不存在的属性</h3><pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript">console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>Object<span class="token punctuation">.</span>foo<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span>
<span class="token keyword">var</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>

<p>访问 Object 对象上的 foo 属性，返回 undefined ， 表示Object 上不存在或者没有定义名为 foo 的属性。数组中的元素在内部也属于对象属性，访问下标就等于访问这个属性，返回 undefined ，就表示数组中不存在这个元素。</p>
<h3 id="函数定义了形参，但没有传递实参"><a href="#函数定义了形参，但没有传递实参" class="headerlink" title="函数定义了形参，但没有传递实参"></a>函数定义了形参，但没有传递实参</h3><pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token comment">//函数定义了形参 a</span>
<span class="token keyword">function</span> <span class="token function">fn</span><span class="token punctuation">(</span><span class="token parameter">a</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span>
<span class="token punctuation">&#125;</span>
<span class="token function">fn</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//未传递实参</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>函数 fn 定义了形参 a， 但 fn 被调用时没有传递参数，因此，fn 运行时的参数 a 就是一个原始的、未被赋值的变量。</p>
<h3 id="使用-void-对表达式求值"><a href="#使用-void-对表达式求值" class="headerlink" title="使用 void 对表达式求值"></a>使用 void 对表达式求值</h3><pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token keyword">void</span> <span class="token number">0</span> <span class="token punctuation">;</span> <span class="token comment">// undefined</span>
<span class="token keyword">void</span> <span class="token boolean">false</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span>
<span class="token keyword">void</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span>
<span class="token keyword">void</span> <span class="token keyword">null</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span>
<span class="token keyword">void</span> <span class="token keyword">function</span> <span class="token function">fn</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span><span class="token punctuation">&#125;</span> <span class="token punctuation">;</span> <span class="token comment">// undefined</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>ECMAScript 明确规定 void 操作符 对任何表达式求值都返回 undefined ，这和函数执行操作后没有返回值的作用是一样的，JavaScript 中的函数都有返回值，当没有 return 操作时，就默认返回一个原始的状态值，这个值就是 undefined，表明函数的返回值未被定义。</p>
<p>因此，undefined 一般都来自于某个表达式最原始的状态值，不是人为操作的结果。当然，你也可以手动给一个变量赋值 undefined，但这样做没有意义，因为一个变量不赋值就是 undefined 。</p>
<h2 id="null"><a href="#null" class="headerlink" title="null"></a>null</h2><p>null 的字面意思是：空值。这个值的语义是，希望表示一个对象被人为的重置为空对象，而非一个变量最原始的状态。在内存里的表示就是，栈中的变量没有指向堆中的内存对象，即：</p>
<p><img  srcset="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%20300%20300'%3E%3C/svg%3E" data-src="https://api2.mubu.com/v3/document_image/d3f2bd56-fb43-4b9f-aaa3-0635e94a0525-3807603.jpg" class="lozad post-image"src="https://api2.mubu.com/v3/document_image/d3f2bd56-fb43-4b9f-aaa3-0635e94a0525-3807603.jpg"></p>
<p>当一个对象被赋值了null 以后，原来的对象在内存中就处于游离状态，GC 会择机回收该对象并释放内存。因此，如果需要释放某个对象，就将变量设置为 null，即表示该对象已经被清空，目前无效状态。试想一下，如果此处把 null 换成 undefined 会不会感到别扭? 显然语义不通，其操作不能正确的表达其想要的行为。</p>
<p>与 null 相关的另外一个问题需要解释一下：</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token keyword">typeof</span> <span class="token keyword">null</span><span class="token operator">==</span> <span class="token string">'object'</span> <span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>null 有属于自己的类型 Null，而不属于Object类型，typeof 之所以会判定为 Object 类型，是因为JavaScript 数据类型在底层都是以二进制的形式表示的，二进制的前三位为 0 会被 typeof 判断为对象类型，而 null 的二进制位恰好都是 0 ，因此，null 被误判断为 Object 类型。</p>
<pre class="line-numbers language-none"><code class="language-none">000 - 对象，数据是对象的应用
1 - 整型，数据是31位带符号整数
010 - 双精度类型，数据是双精度数字
100 - 字符串，数据是字符串
110 - 布尔类型，数据是布尔值<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>其实，我们可以通过另一种方法获取 null 的真实类型：</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">;</span> <span class="token comment">// [object Null]</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>通过 Object 原型上的toString() 方法可以获取到JavaScript 中对象的真实数据类型，当然 undefined 类型也可以通过这种方式来获取：</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span><span class="token keyword">undefined</span><span class="token punctuation">)</span> <span class="token punctuation">;</span> <span class="token comment">// [object Undefined]</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<h2 id="相似性"><a href="#相似性" class="headerlink" title="相似性"></a>相似性</h2><p>虽然 undefined 和 null 的语义和场景不同，但总而言之，它们都表示的是一个<strong>无效的值</strong>。 因此，在JS中对这类值访问属性时，都会得到异常的结果：</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token keyword">null</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Cannot read property 'toString' of null</span>
<span class="token keyword">undefined</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Cannot read property 'toString' of undefined　　</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span></span></code></pre>

<p>ECMAScript 规范认为，既然 null 和  undefined 的行为很相似，并且都表示 一个无效的值，那么它们所表示的内容也具有相似性，即有</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token keyword">undefined</span> <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">;</span> <span class="token comment">// true</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>不要试图通过转换数据类型来解释这个结论，因为：</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token function">Number</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 0</span>
<span class="token function">Number</span><span class="token punctuation">(</span><span class="token keyword">undefined</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// NaN</span>
<span class="token comment">// 在比较相等性之前，null 没有被转换为其他类型</span>
<span class="token keyword">null</span><span class="token operator">==</span> <span class="token number">0</span> <span class="token punctuation">;</span> <span class="token comment">//false</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

<p>但 === 会返回 false ，因为全等操作 === 在比较相等性的时候，不会主动转换分项的数据类型，而两者又不属于同一种类型：</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token keyword">undefined</span> <span class="token operator">===</span> <span class="token keyword">null</span><span class="token punctuation">;</span> <span class="token comment">// false，类型不相同</span>
<span class="token keyword">undefined</span> <span class="token operator">!==</span> <span class="token keyword">null</span><span class="token punctuation">;</span>  <span class="token comment">// true, 类型不相同</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span></span></code></pre>

<h2 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h2><p>用一句话总结两者的区别就是：undefined 表示一个变量自然的、最原始的状态值，而 null 则表示一个变量被人为的设置为空对象，而不是原始状态。所以，在实际使用过程中，为了保证变量所代表的语义，不要对一个变量显式的赋值 undefined，当需要释放一个对象时，直接赋值为 null 即可。</p>
<h2 id="参考链接"><a href="#参考链接" class="headerlink" title="参考链接"></a>参考链接</h2><p><a target="_blank" rel="noopener" href="https://www.cnblogs.com/onepixel/p/7337248.html">详解 undefined 与 null 的区别</a></p>

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